[Test Plan] Interpolated String Improvements

[333fred]

Proposal: ​ https://github.com/dotnet/csharplang/issues/4487

Spec: https://github.com/dotnet/csharplang/blob/main/proposals/csharp-10.0/improved-interpolated-strings.md

Feature Branch: https://github.com/dotnet/roslyn/tree/features/interpolated-string

Compiler:

• [x] LangVersion
• [x] DefaultInterpolatedStringHandler
  • [x] Not present
  • [x] Present
  • [x] Malformed
  • [x] In an async method with no await holes
  • [x] In an async method with await holes
• [x] Binary operator combination of interpolated strings
  • [x] + operator
  • [x] + operator with non interp string
• [x] Custom InterpolatedStringHandler
  • [x] Incorrect ctor
  • [x] implicit conversions to int?
  • [x] Missing AppendLiteral
  • [x] Incorect AppendLiteral
  • [x] Not return bool or void
  • [x] Missing AppendFormatted
  • [x] Incorrect AppendFormatted
  • [x] Not return bool or void
  • [x] Mismatched AppendFormatted and AppendLiteral return types
• [x] Handler overloads
  • [x] Pick handler over regular string for non-const
  • [x] Regular string over handler for const
  • [x] Handler over conversion to string
  • [x] Handler overloads with arguments:
  • [x] val over in
  • [x] val over ref
• [x] Handler args as:
  • [x] val
  • [x] ref
  • [x] in
  • [x] implicit ref (allow mismatch)
  • [x] out
  • [x] normal / expanded form
  • [x] other params after handler param
  • [x] default values
  • [x] reordered at call site
• [x] InterpolatedStringHandlerArgumentAttribute
  • [x] Missing
  • [x] Malformed
• [x] Parameter names
  • [x] Single parameter name
  • [x] Multiple parameters
  • [x] ""
  • [x] Passes this
  • [x] On a static method
  • [x] On a ctor
  • [x] Incorrect parameter name
  • [x] Mix of correct and incorrect param names
  • [x] Referring to self
• [x] Nullable analysis for format holes and handler constructor (issue https://github.com/dotnet/roslyn/issues/54583)
• [x] Await in interpolation hole
  • [ ] Ref parameter
• [x] Dynamic handler
• [x] IOperation
• [x] Control Flow Graph
• [x] Semantic model
  • [x] GetConstantValue on additive expressions

Productivity:

• [ ] Intellisense for methods that have builder arguments
• [ ] Verify experience on some methods/overloads that will get builders (string.Create, Debug.Assert, Debug.Write{Line}If, StringBuilder.Append{Line}, MemoryExtensions.TryWrite)
• [ ] Extract interpolated string to a local (risks changing meaning/perf)
• [ ] Find all refs on custom handler type and constructor, and append methods
• [ ] go to definition

LDM:

• [x] conversion operator to string? (details) (answer: no)
• [x] parentheses and additive expressions (details) (answer: allow parens and suppressions)

[333fred]

Test ideas from review:

• Test with InterpolatedStringHandlerArgumentAttribute on an out parameter.
• Test arguments on delegates, lambdas, and local functions.
• Test with handler having a default argument.
• Test on dictionary initializers.
• Test on extension methods
• Test when an explicit conversion to a handler would fail, but would succeed in an argument position.

[333fred]

https://github.com/dotnet/roslyn/pull/54585#discussion_r665711537 https://github.com/dotnet/roslyn/pull/54585#discussion_r665653862 https://github.com/dotnet/roslyn/pull/54585#discussion_r665648115 https://github.com/dotnet/roslyn/pull/54585#issuecomment-874428443